This invention relates to a method for making piles and to apparatus for practising the method of the invention. A preferred embodiment of the invention provides a method and apparatus for making piles to support the foundation of a structure, such as a building.
Piles are used to support structures, such as buildings, when the soil underlying the structure is too weak to support the structure. There are many techniques that may be used to place a pile. One technique is to cast the pile in place. In this technique, a hole is excavated in the place where the pile is needed and the hole is filled with cement. A problem with this technique is that in weak soils the hole tends to collapse. Therefore, expensive shoring is required. If the hole is more than about 4 to 5 feet deep then safety regulations typically require expensive shoring and other safety precautions to prevent workers from being trapped in the hole.
Turzillo, U.S. Pat. No. 3,962,879 is a modification of this technique. In the Turzillo system a helical auger is used to drill a cylindrical cavity in the earth. The upper end of the auger is held fixed while the auger is rotated about its axis to remove all of the earth from the cylindrical cavity. After the earth has been removed fluid cement water is pumped through the shaft of the auger until the hole is filled with cement. The auger is left in place. Turzillo, U.S. Pat. No. 3,354,657 shows a similar system.
Langenbach Jr., U.S. Pat. No. 4,678,373 discloses a method for supporting a structure in which a piling bearing a footing structure is driven down into the ground by pressing from above with a large hydraulic ram anchored to the structure. The void cleared by the footing structure may optionally be filled by pumping concrete into the void through a channel inside the pile. The ram used to insert the Langenbach Jr. piling is large, heavy and expensive.
Another approach to placing piles is to insert a hollow form in the ground with the piles desired and then to fill the hollow form with fluid cement. Hollow forms may be driven into the ground by impact or screwed into the ground. This approach is cumbersome because the hollow forms are unwieldy and expensive. Examples of this approach are described in U.S. Pat. Nos. 2,326,872 and 2,926,500.
Helical pier systems, such as the CHANCE(trademark) helical pier system available from the A. B. Chance Company of Centralia Mo. U.S.A., provide an attractive alternative to the systems described above. As described in more detail below, the CHANCE helical pier system includes one or more helical screws mounted at the end of a shaft. The helical screw comprises a section of metal plate having its inner edge welded to the shaft. The area around the inner edge is the root region of the screw. The plate is bent so that its outer edge generally follows a helix. The shaft is turned to draw the helical screw downwardly into a body of soil. The screw is screwed downwardly until the screw is seated in a region of soil sufficiently strong to support the weight which will be placed on the pier.
Brackets may be mounted on the upper end of the pier to support the foundation of a building. Helical pier systems have the advantages that they are relatively inexpensive to use and are relatively easy to install in tight quarters. Helical pier systems have two primary disadvantages. Firstly, they rely upon the surrounding soil to support the shaft and to prevent the shaft from bending. In situation where the surrounding soil is very weak or the pier is required to support very large loads the surrounding soil cannot provide the necessary support. Consequently, helical piers can bend in such situations. A second disadvantage of helical piers is that the metal components of the piers are in direct contact with the surrounding soil. Consequently, if the shaft passes through regions in the soil which are highly chemically active then the shaft may be eroded, thereby weakening the pier. A third disadvantage of helical piers exists in piers which comprise large diameter helices which bear large loads. Such helices can buckle and cause the pier to fail. Because their load bearing capacity is limited, helical pier systems have not been able to replace more conventional piles in many applications.
There is a need for a relatively inexpensive method for forming piles without the use of heavy expensive equipment which overcomes at least some of the above-noted disadvantages of helical piers.
This invention provides methods for forming piles which use a screw to pull a soil displacing member through soil. One aspect of the invention provides a method comprising the steps of: providing a screw pier comprising a shaft having a screw proximate a first end thereof and a first soil displacing member projecting radially outwardly from the shaft at a location spaced toward a second end of the shaft from the screw; placing the screw in soil and turning the shaft to draw the screw into the soil thereby causing the screw to pull the first soil displacing member through the soil, thereby clearing soil from a cylindrical region surrounding the shaft; either during or after the step (b) filling the cylindrical region with a fluid grout; and, allowing the fluid grout to solidify, thereby encasing the shaft.
Preferably the step of filling the cylindrical region with fluid grout comprises providing a bath of fluid grout around the shaft at a point where the shaft enters the soil and allowing fluid grout from the bath of fluid grout to flow into the cylindrical region as the screw is turned. A preferred embodiment comprises encasing at least a root portion of the screw in solidified grout. This protects the root portion of the screw from corrosive soils and reinforces the screw. In the preferred embodiment the method includes the steps of removing soil from a volume surrounding at least a root portion of the screw by holding the shaft against longitudinal motion, turning the screw in a first sense and forcing a fluid grout under pressure into the volume; and, allowing the grout in the volume to harden, thereby encasing surfaces of the screw in a protective layer of solidified grout. Preferably the fluid grout is forced under pressure into the volume while the screw is rotating. Most preferably the fluid grout is forced under pressure into the volume by forcing the fluid grout under pressure through a longitudinal channel within the shaft and directing the grout into the volume through apertures in a wall of the shaft.
Another preferred embodiment of the invention provides a method adapted to create a stepped pile. In this method, the screw pier comprises a plurality of additional soil displacing members having diameters larger than a diameter of the first soil displacing member, the additional soil displacing members at spaced apart locations on the portion of the shaft between the second end and the first soil displacing member. The additional soil displacing members toward the second end have diameters larger than diameters of the additional soil displacing members toward the first soil displacing member. The method includes drawing the additional soil displacing members through the soil to stepwise increase a diameter of the cylindrical region.
Another aspect of the invention provides a method for forming a pile. The method comprises the steps of: providing a screw pier comprising a shaft having a screw at one end thereof; placing the screw in the soil and turning the shaft to draw the screw into the soil; when the screw has reached a desired point, removing soil from a volume surrounding the screw by holding the shaft against longitudinal motion and turning the screw; and, forcing a fluid grout under pressure into the volume and allowing the grout in the volume to harden thereby encasing surfaces of the screw in a protective layer of solidified grout.
Yet another aspect of the invention provides a screw pier for making a grout encased stepped pile. The pier comprises an elongated shaft having first and second ends; a screw adjacent the first end of the shaft; a plurality of soil displacing members at spaced apart locations along the shaft, a first one of the soil displacing members having a diameter smaller than a diameter of the screw located near the screw, other ones of the soil displacing members having diameters larger than the first one of the soil displacing members, the soil displacing members nearer to the second end of the shaft having larger diameters than the soil displacing members farther from the second end of the shaft. In a preferred embodiment, the soil displacing members comprise flanges projecting radially from the shaft. The soil displacing members may comprise generally planar disks mounted on and oriented generally perpendicularly to the shaft.
A further aspect of the invention provides a screw pier for making a grout encased pile. The screw pier comprises: a lead section comprising a screw, a head and a soil displacement member between the screw and the head; an elongated shaft having a first end coupled to the lead section head; an elongated drive tool having a socket in driving engagement with the lead section head, the elongated shaft extending through a central bore in the drive tool; and a fastener at a second end of the elongated shaft, the fastener holding the drive tool socket engaged with the lead section head. After placement of the screw pier the drive tool may be removed and re-used. In a preferred embodiment, the drive tool comprises two or more sections connected by one or more joints and each joint comprises a head end of one drive tool section received in a socket on one end of another drive tool section the socket is movable longitudinally relative to the head end between first and second positions. When the socket is in its first position, an edge of the socket projects past an abutment on the head end to provide a recess facing the screw. The recess is capable of receiving tab portions of sectors of a soil displacing member. When the socket is in its second position, the edge of the socket is retracted, thereby releasing the tab portions of the sectors.
The invention also provides a drive tool for installing a grout encased screw pier. The drive tool comprises an elongated shaft penetrated by a central bore. The shaft comprises two or more sections connected by one or more joints. The drive tool has a socket for drivingly coupling to a screw pier lead section at one end of the shaft. Each of the joints comprises a head end of one shaft section slidably received in a socket on one end of another shaft section. The socket is movable longitudinally relative to the head end between first and second positions. When the socket is in its first position, an edge of the socket projects past an abutment on the head end to define a recess facing toward the first end of the shaft. When the socket is in its second position, the edge of the socket does not project past the abutment.